1. Field of the Invention
The present invention relates to a method and an arrangement for supporting vertically depending electrical resistance elements.
2. Description of the Related Art
Such resistance elements are used primarily to heat industrial furnaces or ovens. Each element includes current conducting legs that run downwards and upwards a number of times. The top of the element merges with a number of terminals that are connected to one more sources of electric current. The element thus hangs from the roof of the furnace and extends downwards in operation. The legs are subjected to strong thermal variations in operation, due to the power developed therein. This variation results in bending or twisting of individual legs in the element as the temperature changes. Consequently, the element is provided along its length with a number of ceramic discs that include through-penetrating holes through which respective element legs extend. These ceramic discs are intended to hold the legs of the element apart and out of contact with one another. Mutual contact of the legs would cause the element to short circuit, resulting in serious damage, if not destruction, of the resistance element.
The uppermost ceramic disc or the uppermost discs may also serve to support the weight of the resistance element. According to the present state of the art, this is achieved by coupling pairs of legs together with the aid of current conducting plates which rest on the uppermost ceramic disc or on the uppermost discs, depending on the geometry of the resistance element concerned. Legs thus extend pair-wise through a ceramic suspension disc and are joined together on the upper side of said disc through the medium of such a current conducting plate and supported in this way by the ceramic disc.
The power developed in the legs is often very high. Typical powers developed in the legs of a resistance element in industrial operation can be in the order of 20-50 kW. The resistance element is often driven cyclically, meaning that the temperature in the vicinity of the ceramic plates will vary over a wide temperature range in the space of time.
This heavy thermal load in combination with the mechanical load borne by the supportive ceramic discs results in the formation of cracks in said discs and finally in fracturing of the discs. When this occurs, the resistance element will no longer be supported by the broken discs and will collapse down into the furnace, thereby resulting in significant repair costs.
A typical life span of a supporting ceramic disc is of the order of three to six months.
An industrial furnace may include a considerable number of resistance elements, for example several hundred. This means that serious costs are often incurred in changing or replacing supportive ceramic discs. It is therefore desirable to find a way of increasing the useful length of life of such discs.